Cellular communication systems continue to grow in popularity and have become an integral part of both personal and business communications. Cellular telephones and similar devices allow users to place and receive phone calls most anywhere they travel. Moreover, as cellular telephone technology is increased, so too has the functionality of cellular devices. For example, many cellular devices now incorporate Personal Digital Assistant (PDA) features such as calendars, address books, task lists, calculators, memo and writing programs, etc. These multi-function devices usually allow users to send and receive electronic mail (email) messages wirelessly and access the internet via a cellular network and/or a wireless local area network (WLAN), for example.
Many of the cellular communications use packet burst transmissions as part of a Global System for Mobile communications (GSM) system, which includes the 450 MHz, 900 MHz, 1800 MHz and 1900 MHz frequency bands. The current generation of wireless transceivers typically use two main types of receiver architectures, i.e., a direct conversion receiver architecture or a digital low-IF receiver architecture (also termed very low-IF, i.e., VLIF), thus, eliminating much of the prior generation analog down conversion stage. Much of the expensive and bulky intermediate frequency (IF) components used in conventional superheterodyne receivers has been eliminated with direct conversion and low-IF receiver architecture. In a direct conversion receiver, a signal is converted directly to baseband, while in a digital low-IF receiver, some advantages of the superheterodyne remain with the economic and integrated advantages of a direct conversion receiver.
In a low-IF receiver, on the other hand, the RF signal can be mixed down to a non-zero low or moderate intermediate frequency, typically a few megahertz in some examples. Thus, the low-IF receiver architecture includes many of the desirable properties of the zero-IF receiver architectures, yet still avoids DC offset and some 1/F-noise problems. The non-zero IF receiver architecture will reintroduce some signal image issues. In a low-IF receiver, the RF signal is band selected and downconverted to the frequency close to baseband, sometimes as close as 100 KHz. This low-IF signal can be filtered with a low pass filter and amplifier before its conversion to the digital domain by an analog-to-digital converter (ADC). Any final signal downconversion for baseband and fine gain control can be performed digitally in a processor
It is also possible to incorporate some high-resolution, oversampling and delta-sigma converters to permit channel filtering, including the use of digital signal processing (DSP) techniques rather than analog filters. The signal could interface to a digital processor or a digital-to-analog converter and output analog Inphase (I) and Quadrature (Q) signals to the processor.
An important GSM receiver parameter is the rejection of interferer signals to enhance performance of the low-IF receiver. European Telecommunications Standards Institute (ESTI) mobile station conformance specifies very strict certification tests (14.5, 14.18.3) that are not easy to pass using GSM receivers currently available on the market. Even if a receiver passes certification tests, having better performance resulting from interferer signal rejection may significantly improve end-user experience in large urban areas, where strong radio interference is a common problem.
Rejection of interferer signals is usually a problem for very low-IF receiver architecture where rejection performance is limited by I/Q gain and phase imbalance. The common way to address this problem is to use I/Q imbalance calibration where I/Q gain and phase will be adjusted during digital baseband processing based on previously calculated calibration tables. Unfortunately, these types of calibration processes do not provide enough accuracy when time measurements during calibration cycle are limited. If the calibration time is extended, however, the manufacturing costs for a single communications device may increase significantly.
Some proposals to solve such problems toggle the local oscillator (LO) with a “round-robin” scheme using a low-side LO injection during one receive (RX) session and a high-side LO injection in another session. This solution does not use any feedback from the radio environment, thus achieving only a basic “averaging” of the interferer image. There is typically no knowledge of the external environment. Also, by not taking the type of signal used in the receive session into account, results in a worst case scenario when, for example, distorted signal bursts are mixed with Received Signal Strength Indication (RSSI) measurements, and the resulting data stream picks the worst interferer appearance.